The Nasa image on left was taken from about 18,000 miles away; the one on the right is a far more detailed image.
Photograph: AP

Nasa’s New Horizons spacecraft has beamed home its first close-up images of Ultima Thule, a lump of rock the shape of an unfinished snowman that lies 4 billion miles away on the edge of the solar system.

Taken as the probe sped past the body in the early hours of New Year’s Day, the pictures reveal a dark reddish object about 21 miles long and 10 miles wide that spins on its axis once every 15 hours or so. The colour image of Ultima Thule, revealing its reddish tint, was taken at 05.01 GMT on New Year’s Day from a distance of about 18,000 miles, 30 minutes before the probe made its closest pass of the space rock.

The spacecraft snapped thousands of images of the object, known formally as 2014 MU69, in a fleeting encounter that set a record for the most distant flyby in history. From a billion miles beyond Pluto, it takes data sent at the speed of light about six hours to reach Earth.

“Meet Ultima Thule,” said Alan Stern, the mission’s principal investigator, as he unveiled the images at a press conference on Wednesday. The scientists originally described the object as shaped like a bowling pin, but Stern said he had changed his mind on seeing the new picture. “That bowling pin is gone. It’s a snowman if anything at all,” he said.

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The odd shape of Ultima Thule is thought to have come about when swirling ice and dust particles coalesced in the early life of the solar system and eventually led to two large lumps of rock colliding and sticking together. Stern said that the gravity of each “lobe” was enough to keep the two parts of Ultima Thule in contact.

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New Horizons beams home close-ups of Ultima Thule – video

Preliminary analysis of the images showed that the neck that joins the two lobes of Ultima Thule is brighter than the rest of the mottled surface, probably because loose grains had collected there, said Cathy Olkin, a scientist on the mission. The dark red hue of much of the surface is thought to be due to the effects of space radiation on exotic ices on the surface.

The New Horizons spacecraft launched in 2006 on a mission to explore Pluto. When it shot past the dwarf planet in 2015, it captured breathtaking shots of the distant world, one now known to host mountains of solid nitrogen and volcanoes that blast ice into space.

Like Pluto, Ultima Thule lies in a region of the solar system called the Kuiper belt, a doughnut-shaped ring of dwarf planets, boulders and other debris left over from the formation of the solar system 4.6 billion years ago. The Nasa probe came within 2,200 miles of Ultima Thule, meaning “beyond the known world”, as it hurtled past at 31,500mph.

Mission scientists at Johns Hopkins University in Maryland had to wait 10 hours for New Horizons to confirm that the flyby at 05.33 GMT on Tuesday had proceeded smoothly and that the spacecraft was still operational. At such terrific speed, a collision with a particle as small as a grain of rice could have spelt disaster for the probe.

At a press briefing on Tuesday to mark the flyby, team members released images of Ultima Thule taken when New Horizons was still half a million miles away. It was described by project scientist Hal Weaver as “a pixelated blob”, and at the time image analysts did not rule out it being two small bodies in orbit around one another.

The early images solved one mystery surrounding the distant space rock. Elongated bodies like Ultima Thule are expected to brighten and fade with clockwork regularity as they tumble through space. This is because the body’s long side reflects more light than its short side. But Ultima Thule shows no such variation in brightness. The reason is that the rock is spinning like a propeller on an axis that points towards Earth, meaning it reflects the same amount of light in our direction all the time.

Scientists on the mission believe that studying Ultima Thule could provide unprecedented insights into the conditions that prevailed in our cosmic neighbourhood more than four and a half billion years ago. Kuiper belt objects are thought to have occupied their distant positions since the earliest days of the solar system and may look the same today as they did back then.

If all goes well, the spacecraft will beam home more data over the next 20 months. The highest-resolution images from the flyby are expected in February.